Photothermographic element and process

ABSTRACT

Certain acrylamide polymers in a layer contiguous to a photothermographic layer comprising (a) photographic silver halide in association with (b) a silver salt of certain heterocyclic thione compounds, (c) an organic reducing agent, and (d) a polymeric, synthetic binder for the photothermographic layer of a photothermographic element provide, for example, increased stability prior to imagewise exposure without significantly adversely affecting sensitometric properties of the photothermographic element. The acrylamide polymers can comprise an overcoat layer or a layer between the photothermographic layer and a support for the photothermographic element or can be in both such layers.

United States Patent n91 Sutton et al.

[ PHOTOTHERMOGRAPHIC ELEMENT AND PROCESS [75] Inventors: Richard C.Sutton; Heinz E.

Stapelfeldt, both of Rochester, NY.

[73] Assignee: Eastman Kodak Company,

Rochester, NY.

[22] Filed: July 16, 1974 [21] Appl. No.: 489,029

[52] U.S. Cl. 96/66 T; 96/48 HD; 96/76 R;

96/1 14.1 [51] Int. Cl. G030 5/30; G03c l/04; G03c H72 [58] Field ofSearch 96/48 HD, 66 T, 76 R, 95,

[451 July 8,1975

[/1974 Sullivan et al. 96/1 14.! 12/1974 Hamb et al. 96]! 14.1

[57] ABSTRACT Certain acrylamide polymers in a layer contiguous to aphotothermographic layer comprising (a) photographic silver halide inassociation with (b) a silver salt of certain heterocyclic thionecompounds, (c) an organic reducing agent, and (d) a polymeric, syntheticbinder for the photothermographic layer of a photothermographic elementprovide, for example, increased stability prior to imagewise exposurewithout significantly adversely affecting sensitometric properties ofthe photothermographic element. The acrylamide polymers can comprise anovercoat layer or a layer between the photothermographic layer and asupport for the photothermographic element or can be in both suchlayers.

17 Claims, No Drawings PHOTOTHERMOGRAPHIC ELEMENT AND PROCESS BACKGROUNDOF THE INVENTION Field of the Invention This invention relates tocertain acrylamide polymers in certain photothermographic materialscomprising photographic silver halide in association with a silver saltof certain heterocyclic thione compounds. In one of its aspects, itrelates to a photothermographic element comprising a support havingthereon (I) a layer comprising the described silver salts of aheterocyclic thione and a contiguous layer (II) comprising certainacrylamide copolymers. In another of its aspects, it relates to a methodof developing a latent image in a photothermographic element comprisingthe described acrylamide polymers.

Description of the State of the Art Phototherrnographic materialscomprising photographic silver halide in association with a silver saltof certain heterocyclic thione compounds and an organic reducing agentare known in the art. These photothermographic materials are described,for example, in US. Pat. No. 3,785,830 issued Jan. I5, I974. One problemwhich has been encountered with these photothermographic materials isthe need for increased preprocessing stability of the photothermographicelement. This problem is illustrated in the following comparativeExample 2. This problem is illustrated by failure of thephotothermographic material to provide a desired developed image afterstorage at elevated temperatures such as storage at 38 C. at 50%relative hu midity.

It has been proposed in some cases to provide the photothermographicmaterial with a protective layer, such as a protective overcoat layer. Acommonly employed overcoat layer for photothermographic ele mentscomprises cellulose acetate. Such an overcoat layer is described, forexample, in Belgian Pat. No. 729,043 and US. Pat. No. 2,732,304 issuedJan. 24, I956. Such an overcoat comprising cellulose acetate does notprovide a useful solution to problems encountered withphotothermographic materials comprising photographic silver halide inassociation with silver salts of certain heterocyclic thiones asdescribed in US. Pat. No. 3,785,830 issued Jan. 15, I974.

Polymer overcoat layers have been proposed for photothermographicelements to reduce susceptibility to abrasion marks, especially inmachine processing wherein the photothermographic layer side of theelement is contacted with a metal roller or the like. In ad dition, theprocessing of photothermographic elements by contacting thephotothermographic element with a heating means can cause undesiredphysical properties such as surface cracking, reticulation and bubblingwhich can detract from the overall image quality desired in thephotothermographic element. An ethyl cellulose overcoat layer and otherovercoat layers have not satisfactorily overcome these problems in aphotothermographic element as described.

In the application of polymeric layers, i.e., either overcoat layers orlayers between the support and the photothermographic layer, it isnecessary to employ polymers which are resistant to decomposition orother undesired effects at the processing temperatures employed, such astemperatures above about 100 C,

Polymers, in order to be satisfactory with the photothermographicmaterials as described, must satisfy each of the followingcharacteristics in addition to those described: (1) they providesufficient resistance to abrasion and fingerprint marking to enablemachine processing, (2) they provide sufficient resistance toreticulation and surface cracking upon processing with heat in aphotothermographic element, (3) they provide sufficient resistance tosurface bubbling in a photothermographic material upon processing withheat, (4) they do not significantly adversely afiect sen sitometricproperties of the photothermographic materials, and (5) they aresufficiently transparent for desired viewing of an image. Although thereare many polymers that are resistant to high-temperature decomposition,these polymers, as a class, are not satisfactory for use with thedescribed photothermographic materials because properties are requiredother than the property of resistance to decomposition at hightemperatures. This is illustrated by appended comparative examples.

There has, accordingly, been a continuing need to provide improvedphotothermographic materials comprising photographic silver halide inassociation with silver salts of certain heterocyclic thione compounds,as described, to provide the desired described properties such asimproved stability prior to imagewise exposure, resistance to abrasionmarks, fingerprint marks and undesired surface properties, withoutsignificantly adversely affecting sensitometric properties of thephotothermographic material.

Summary of the Invention It has been found, according to this invention,that the described properties are provided in a photothermographicelement comprising a support having thereon a layer comprising (a)photographic silver halide in association with (b) a silver salt of aheterocyclic thione as described herein, (c) an organic reducing agentfor the silver salt of the heterocyclic thione, and (d) a polymeric,synthetic binder for layer (I) and a polymer layer (II), contiguous tolayer (I), wherein the polymer layer (II) comprises at least 50% byweight of a polymer (A) comprising the repeating units represented bythe formulas:

l +CH C|H+ and +cn, c+

CI'=O R2 NH2 wherein:

R is hydrogen or alkyl containing l to 4 carbon atoms;

or 6-membered azonia nitrogen-containing ring such as an imidazoliurnsalt group, such as a l-aza- 3-methyl-3-azoniacyclopenta-2,4-diene-l-ylmethosulfate salt group, or a pyridinium salt group, such asl,2-dimethyl-l-azonia-S-phenyl methosulfate salt group, and

the weight ratio of starting monomers for the units B and C is,respectively, about 60:40 to 100:0.

The described polymers are acrylamide polymers. The acrylamide polymersare preferably copolymers which exhibit reduced or no adverse physicalor chemical properties resulting from the thermal processing describedherein. This is intended to mean that the described polymers arepreferably copolymers which do not adversely discolor, decompose,crystallize, flow or the like as a result of the heating describedherein.

Also, according to the invention, a process is provided for developingan image in the described photo thermographic element by uniformlyheating the element, such as from about 80 to about 200 C., for asufficient time to provide the desired developed image. This process canbe carried out by contacting the photothermographic element with asuitable heating means to provide the described temperature.

Detailed Description of the Invention Various acrylamide polymers withinthe designated formula are useful in photothermographic materialsaccording to the invention.

Useful acrylamide polymers do not adversely flow, smear or distort atthe processing temperatures for a described photothermographic material.The acrylamide polymers also do not cause adverse opacification of thephotothermographic material. Useful acrylamide polymers have an averagemolecular weight of at least about 50,000, and preferably from about100,000 to about 2,000,000. The molecular weight can be determined bymethods known in the polymer art. Useful acrylamide polymers haveinherent viscosities, measured at a concentration of 0.25 g. perdeciliter in 1 normal sodium chloride solution at 25 C., ranging from0.2 to 2.0, and preferably from 0.6 to 17.

Useful acrylamide polymers are typically transparent and colorless. Itis necessary, if the polymer is not completely transparent, that it beat least transparent to the wavelength of radiation employed to providea latent image in the photothermographic element of the invention whenexposure is through a layer comprising the acrylamide polymer.

Some acrylamide polymers, especially acrylamide copolymers, useful inphotothermographic elements according to the invention are described in,for example, U.S. Pat. No. 3,658,878 of Smith issued Apr. 25, 1972. U.S.Pat. No. 3,591,386 of Abbott et al. issued July 6, 1971, U.S. Pat. No.3,488,708 of Smith issued Jan, 6, 1970, U.S. Pat. No. 3,459,790 of Smithissued Aug. 5, 1969, and U.S. Pat. No. 3,554,987 of of Smith issued Jan.12, 1971.

Useful acrylamide polymers, especially acrylamide copolymers, can beprepared empolying procedures known in the polymer art, such asdescribed in the above U.S. patents. Generally, any of the known polymerization procedures, particularly copolymerization procedures used inthe polymer art, are useful for making the described acrylamidepolymers.

Typical reactants employed when preparing the polymers within thedescribed structure (A) are acrylamide and acrylamide copolymerized withone of the following monomers:

1 -vinylimidazole 2-methy1- l -vinylimidazole B-methyl- 1vinylimidazo1ium methosulfate N-methylolacrylamide 2-acetoacetoxyethylmethacrylate acrylic acid l-vinyl-2-pyrrolidone 2-methyl-5-vinylpyridine1,Z-dimethyl-S-vinylpyridinium methosulfate The described acrylamidepolymers do not, in the absence of interfering components, significantlyadversely affect the sensitometric properties of the describedphotothermographic materials, such as minimum density, maximum density,photographic speed and the like, whereas cellulose acetate, a commonlyemployed overcoat, causes significant undesired changes in sensitometricbehavior. The described acrylamide polymers also provide desiredincreased sta bility prior to imagewise exposure of a describedphotothermographic element. This is illustrated in the followingexamples.

Examples of useful acrylamide polymers according to the inventioninclude:

Polymer No, Polymer l poly(acrylamide-co'1-vinylimidazole)( weight ratio:10) 11 poly( acrylamideco-Z-methyll -vinylimidazole weight ratio 90:10)poly( acrylamide-co-S-methyll -vinylimidazolium methosulfate weightratio 90: 10) gglygascrylarnideco-N-methylo1acrylamide weight ratio VlllIX poly( acrylamide-col ,Z-dimethyl-S-vinyl-pyridiniummethosulfate)(weight ratio 88: 1 2) X1poly(acrylamide-co-2-vinylpyridine)(weight ratio 89:1 l)

The concentration of acrylamide polymer which is useful in a layer of aphotothermographic element according to this invention can vary,depending upon such factors as the particular photothermographic element, processing conditions, components in the photothermographicelement, particular acrylamide polymer and the like. A usefulconcentration range or coating coverage, is about 0.1 g. to about 1.08g. of acrylamide polymer/m. of support of the photothermographicelement. A useful concentration of acrylamide polymer, when theacrylamide polymer is used as an overcoat layer, is about 0.3 g. toabout 7.5 g., such as about 0.40 g. to about 2.15 g., of acrylamidepolymer/m. of support of the photothermographic element. When theacrylamide polymer is used as a so-called undercoat, that is, a coatingon the support between the support and the photothermographic layer, auseful concentration of acrylamide polymer is about 0.3 g. to about 5.0g., such as about 2.15 g. of acrylamide polymer/m. of support of thephotothermographic element.

The described acrylamide polymer comprises at least 50% by weight of thelayer contiguous the photothermographic layer (1) as described. Otherpolymers can be useful with the acrylamide copolymers. However,typically, the acrylamide polymer comprises about 100% of the contiguouslayer. Other polymers which can be useful with the acrylamide polymersinclude, for example, poly(vinyl alcohol) and polymers having propertiessimilar to poly(vinyl alcohol) such as other acrylamide andN-substituted acrylamide copolymers including. for instance,poly-(acrylamide-co-acrylic acid), and cellulosic materials such assulfoethyl cellulose and sodium cellulose sulfate. Photothermographicelements according to this invention can comprise, if desired, multiplepolymer-containing layers. For example, the photothermographic elementcan comprise an overcoat layer containing an acrylamide polymer asdescribed and an additional overcoat layer comprising another polymersuch as poly(vinyl alcohol).

One embodiment of the invention is in a photother mographic elementcomprising a support having thereon (l):

a. photographic silver halide in association with b. a silver salt of aheterocyclic thione, said heterocy clic thione being represented by theformula:

Z-COOH wherein R represents the atoms completing a 5- memberheterocyclic nucleus and Z is alkylene containing l to 30 carbon atoms,typically 1 to carbon atoms,

0. an organic reducing agent for said silver salt ofa heterocyclicthione, and

d. a polymeric, synthetic binder and,

contiguous to (I), at least one polymer layer (ll), the improvementbeing one wherein said polymer layer (ll) comprises at least 50% byweight of a polymer (A) comprising the repeating units represented bythe formulas:

wherein R is hydrogen or alkyl containing l to 4 carbon atoms, R is asdescribed, and the weight ratio of starting monomers for the units B andC are respectively about 60:40 to l00:0.

Various photographic silver halides can be employed in the describedphotothermographic element. The concentration of photographic silverhalide which is useful in a photothermographic element of the inventioncan be very low compared with photographic materials which containphotographic silver halide in the absence of the other components of thephotothermographic element of the invention. For example, theconcentration of photographic silver halide which is suitable in aphotothermographic element of the inven tion, can be about 0.0025 toabout 0.3 mole of photographic silver halide per mole of silver as thesilver salt of the described heterocyclic thione. In a photothermo'graphic element of the invention, the concentration of photographicsilver halide is typically about 0. l0 times 10' to about 1.29 times 10'moles of photographic silver halide/m of support. Useful photographicsilver halides include, for example, silver chloride, silver bromide,silver iodide, silver bromoiodide, silver chloro bromoiodide, silveriodide, or mixtures thereof. The photographic silver halide can becoarseor fine-grain, very fine-grain silver halide being especiallyuseful. The photographic silver halide can be prepared by any of theknown procedures employed in the photographic art. The silver halide canbe prepared, for example, employing single-jet preparation techniques ordouble-jet preparation techniques such as techniques employed inpreparing Lippmann emulsions and the like. Surfaceimage silver halidecan be useful. If desired, mixtures of surfaceand internal-image silverhalide can be used. Negative-type silver halide is typically employed.The silver halide can be regulargrain silver halide such as described inKlein and Moisar, Journal of Photographic Science, Vol. 12, No. 5,Sept-Oct, l964. pp. 242-251. Photographic silver iodide is especiallyuseful as the photographic silver halide.

The photographic silver halide according to this invention can beunwashed or washed, and can be chemically sensitized employingtechniques empolyed in the photographic art.

it is believed that the latent image formed in the photographic silverhalide upon imagewise exposure of the photothermographic materialincreases the reaction rate between the components of thephotothermographic material upon heating of the photothermo graphicmaterial. It is believed this enables a lower processing temperature tobe employed for developing an image which otherwise would not bepossible.

The term photographic silver halide in association with is intended tomean the photographic silver halide is in a location with respect to theother described components of the photothermographic material whichenables this desired lower processing temperature and provides a moreuseful developed image.

Various silver salts of a heterocyclic thione are useful in thedescribed photothermographic materials of the invention. A useful silversalt of a heterocyclic thione is a silver salt of a heterocyclic thionerepresented by the formula (XX) as described. Selection of an optimumheterocyclic thione silver salt will depend upon such factors as theparticular photothermographic material, particular toning agent,processing temperature, desired image and the like. Examples of useful5- member heterocyclic nuclei containing the described carboxyalkylgroup are thiazoline-Z-thione, benzothiazoline-Z-thione,imida2oline-2-thione, oxazoline-Z- thione, or similar heterocyclicthione nuclei. The heter ocyclic thione nucleus can be substituted withgroups which do not adversely affect the photothermographic propertiesof the photothermographic element of the invention, such as alkylcontaining l to 3 carbon atoms, or phenyl.

especially useful thione compounds within formula (XX) arethiazoline-Z-thiones represented by the formula:

wherein Z is alkylene containing 1 to 4 carbon atoms; and R and R areeach, independently, hydrogen, alkyl containing l to 4 carbon atoms,such as methyl, ethyl, propyl or butyl, or aryl containing 6 to 10carbon atoms, such as phenyl or tolyl, or taken together are the atomsnecessary to complete a benzo group represented by the broken linebetween R and R Other useful heterocyclic thiones within formula (XX)are imidazoline-Z-thiones represented by the formula:

wherein Z is alkylene containing 1 to 4 carbon atoms; R and R are each,independently, hydrogen, alkyl containing 1 to 4 carbon atoms, such asmethyl, ethyl, propyl or butyl, or aryl containing 6 to 10 carbon atoms,such as phenyl or tolyl, or taken together are the atoms necessary tocomplete a benzo group represented by the broken line between R and R";and R is alkyl, typically alkyl containing 1 to 3 carbon atoms, such asmethyl, ethyl or propyl, aryl containing 6 to carbon atoms, such asphenyl, or carboxyalkyl such as carboxyalkyl containin l to 3 carbonatoms, for example, carboxymethyl and carboxyethyl. Other usefulheterocyclic thiones within structure (XX) are oxazoline- 2-thionesrepresented by the formula:

xxm I\RJ N/ t Z'COOH wherein Z, R and R are as defined.

In the definition of Z, as well as Z, Z and Z", as employed herein,alkylene includes straight-chain alkylene and branched-chain alkylene,such as:

CH CH CH Examples of useful thione compounds within the describedformulae include:

3-( 2-carboxyethyl 4-methyl-4-thiazoline-2-thione 3-( Z-carboxyethyl)benzothiazoline-2-thione 3-( 2-carboxyethyl )-5-phenyll,3,4-oxadiazoline-2- thione 3-( 2-carboxyethyl )-5-phenyll,3,4-thiadiazoline-2- thione 3-carboxymethyl-4-methyl-4thiazoline-2-thione 3-( 2-carboxyethyl) l -phenyl-1 ,3,4-triazoline-2-thione l,3-bis( 2-carboxyethyl)imidazoline-Z-thione l,3-bis(Z-carboxyethyl)benzimidazoline-Z-thione 3-( 2-carboxyethyl l-methylimidazoline-2-thione 3-( 2-carboxyethyl )benzoxazoline-Z-thione3-( l-carboxyethyl )-4-methyl-4-thia2oline-2-thione The silver salt ofthe described thione can be prepared directly in the photothermographiccomposition by combining a source of silver, such as silvertrifluoroacetate, with the thione compound in the composi tion. or thesilver salts can be preformed and isolated before addition to thephotothermographic composition. The described thione compounds can beprepared employing processes known in the art. It is desirable to avoidpreparation of the silver salt in the presence of compounds which couldcause reduction.

Preparation of the thione compounds can be carried out employingprocedures described, for example, in an article of R. W. Lamon and W.J. Humphlett, Journal of Heleroc'yclic Chemistry, Vol. 4, pp. 605609,1967, or as described in Belgian Pat. No. 739,705. Preparation of4-thiazoline-2-thiones bearing a carboxyalkyl group in the 3 positioncan, for instance, be effected by treating a dithiocarbamic acid derivedfrom an amino acid and carbon disulfide with an a-halogenated ketone. Inthis process the use of methyl alcohol as a solvent can improve thesolubility of the reactants.

The preparation of the silver salt of 3-carboxymethyl-4-methyl-4-thiazoline-2-thione is typical. This silver complex isprepared by mixing the described thiazoline-2-thione with silvertrifluoroacetate in water and thoroughly dispersing the reactants.Concentrations of the reactants can be varied to provide the desiredratio of silver to heterocyclic compound. Typically, the ratio of thethione compound to silver ion is less than about 2: l The resultingsilver salt can be purified and stored for later mixture with othercomponents of the described photothermographic materials. Dispersing ofthe silver trifluoroacetate with the heterocyclic compound is typicallycarried out at about 38 to about 71 Various reducing agents can beemployed in the photothermographic materials of the invention. These aretypically silver halide developing agents and include, for example,polyhydroxybenzenes such as hydroquinones including, for instance,hydroquinone; alkyl-substituted hydroquinones such astertiarybutylhydroquinone, methylhydroquinone, 2,5-dimethylhy droquinoneand 2,6-dimethylhydroquinone; catechols and pyrogallol; halo-substitutedhydroquinones such as chlorohydroquinone or dichlorohydroquinone;alkoxysubstituted hydroquinones such as methoxyhydroquinone orethoxyhydroquinone; and the like. Other silver halide developing agentswhich can be employed as reducing agents include reductones such asanhydro dihydro piperidino hexose reductone; hydroxytetronic acids andhydroxytetronimides; 3-pyrazolidones such as l-phenyl- 3-pyrazolidone4-methyl-4- hydroxymethyl l -phenyl-3-pyrazolidone, and those describedin British Pat. No. 930,572 published July 3, 1963; hydroxylamines;ascorbic acids such as ascorbic acid, ascorbic acid ketals, and otherascorbic acid derivatives; phenylenediamines; aminophenols; and thelike. Combinations of reducing agents can also be employed. A suitablereducing agent typically is one which in the photothermographiccompositions of the invention provides a developed image within aboutsec. at a temperature of about l00 to 250 C.

A range of concentrations of each component is useful in the describedphotothermographic material. Typically, a photothermographic elementaccording to the invention can comprise a support having thereon in thephotothermographic layer (A) about 0.10 times l0 to about 1.29 times l0moles of photographic silver halide in association with (B) about 2.69times ID to about 21.5 times 10 moles of reducing agent and (C) about2.69 times 10 to about 2l .5 times 10 moles of silver as the describedcomplex per m of support. An optimum concentration of each componentwill depend upon such factors as the particular components, the desiredimage, processing temperature and the like.

If desired, one or more components of the photothermographic element canbe present in one or more lay ers of the element. For example. in somecases it can be desirable to include certain percentages of the reducingagent, silver salt of the described heterocyclic thione and/orphotographic silver halide in the described layer (ll) comprising anacrylamide polymer. This can reduce, for example, migration of certainaddenda throughout the layers of the photothermographic element.

Suitable binders for photothermographic layer (1) can be hydrophilic orhydrophobic, transparent or translucent, and include synthetic polymericmaterials useful in the described photothermographic materials. It isdesirable in many cases to use an acrylamide polymer both as a componentof polymer layer (11) and as a binder for layer (1). Useful acrylamidepolymers for this purpose are within the described formulae.

Various supports are useful for the described photothermographicelement. Typical supports include those film supports which canwithstand processing temperatu res employed for developing an image in aphotothermographic element of the invention. Such film supports include,for example, cellulose ester film, poly(vinyl acetal) film, polystyrenefilm, poly(ethylene terephthaL ate) film, polycarbonate film, filmsupports as described in U.S. Pat. No. 3,634,089 of Hamb issued Jan. ll,1972, and U.S. Pat. No. 3,725,070 of Hamb et al. issued Apr. 3, 1973,and related films or resinous materials. Other useful supports includeglass, paper, metal and the like. Typically, a flexible support isemployed.

The photothermographic elements according to the invention can containaddenda commonly employed in photothermographic elements, such asantistatic and- /or conducting layers, plasticizers, lubricants,surfactants, matting agents, sensitizing dyes, brightening agents,light-absorbing materials, filter dyes, antihalation dyes, absorbingdyes and the like.

If desired, a toning agent can be employed in the photothermographicelement of the invention to provide a desired image. Useful toningagents include, for example, certain heterocyclic compounds which canprovide a more neutral tone image. Examples of useful toning agentsinclude 3-mercapto-1,2,4-triazole and 2,4dimercaptopyrimidine, describedin copending U.S. application Ser. No. 466,331, of White, filed May 2,1974.

The various components of the photothermographic materials of theinvention can be added from water solutions, or suitable organicsolvents can be useful to aid in addition. The components can be mixedusing various procedures known in the photographic art.

It is desirable in some cases to incorporate a hardener, especially analdehyde hardener like formaldehyde, into the described acrylamidepolymer materials. This can provide improved incubation stability. Arange of concentration of the aldehyde hardener can be employeddepending upon such factors as the particular acrylamide polymer,particular components of the photothermographic element, desired image,desired stability and the like. Typically, a concentration of about 0.lto about such as about 0.1 to about 5%, by weight of aldehyde hardener,particularly formaldehyde, is incorporated in the acrylamide polymerbefore coating on the photothermographic element. In some cases, it isdesirable to incorporate the aldehyde hardener in the acrylamide polymerwhen the acrylamide polymer is coated directly on the support beforecoating of the photothermographic layer. The aldehyde hardener, however,can be employed in any one or more of the layers of thephotothermographic element.

In some cases, improved incubation stability is observed when thephotothermographic element containing the aldehyde hardener is storedfor a period of time, such as about at least 2 days, before imagewiseexposure and processing as described. It is believed that this storagetime permits a crosslinking of the polymer in the photothermographicelement. However, the exact mechanism which results in the improvedincubation stability is not fully understood.

An especially useful embodiment of the invention comprises aphotothermographic element comprising a support having thereon,respectively, (A) a layer comprising about 1.08 g./m. of support ofpoly(acrylamide-co-l-vinylimidazole) (weight ratio :10) containing 1% byweight of formaldehyde, (B) a photothermographic layer containingphotographic silver iodide in association with a silver salt of aheterocyclic thione as described, a hydroquinone reducing agent for thesilver salt of the heterocyclic thione, and a synthetic polymeric bindercomprising poly(acrylamide-co-Z- acetoacetoxyethyl mmethacrylate)(weight ratio 98:2) or poly(acrylamide-co-l-vinylimidazole) (weightratio 90:10), and (C) an overcoat layer comprising about L08 g./m. ofsupport of poly(acrylamide-co-lvinylimidazole (weight ratio 90:10).

A range of concentration of the described acrylamide polymer can beuseful in an overcoat on the described photothermographic elementaccording to the invention. While such factors as the particularphotothermographic materials, the particular acrylamide copolymer,processing temperature and the like will influence the optimumconcentration range of acrylamide copolymer in the overcoat layer,typically a concentration of about 0.3 to about 7.5 g., such as about0.4 to about 2.l5 g., of acrylamide copolymer in the overcoat layer perm. of support is useful.

In some cases, it is desirable to incorporate a portion of the describedreducing agent in the acrylamide copolymer layers as described. Whilethe concentration which is most useful of the described reducing agentwill depend upon the described factors, such as particular reducingagent, desired image, processing temperature, particular acrylamidepolymer and the like, a useful reducing agent concentration range in theacrylamide copolymer when the acrylamide copolymer is used as anovercoat is typically about 0.25 to about 0.55 g. of reducing agent perm. of support. When the reducing agent is employed in an acrylamidepolymer layer between the support and the photothermographic layer,typically a concentration of about 2.15 to about 4.30 g. of reducingagent is used per m? of support.

Various imagewise-exposure means are useful with the photothermographicmaterials according to the invention. Photothermographic materialsaccording to the invention are typically sensitive to the ultravioletand blue regions of the spectrum and exposure means which provide thisradiation are preferred. Typically, a photothermographic elementaccording to the invention is exposed imagewise with a visible lightsource such as a tungsten lamp.

A visible image can be developed in a photothermographic element asdescribed, after imagewise exposure, within a short time by overallheating of the photothermographic element. For example, thephotothermographic element can be overall heated for about 1 to about 90sec. at a temperature of about 100 to about 200 C., preferably about l40to about l70 C. Usually, the time of heating is less than about 20 sec.,such as about l to about 4 sec., at a temperature of about l5O to about170 C. Increasing or decreasing the length of time of heating can enableuse of a lower or higher temperature within the described range.

Any suitable means can be useful for providing the desired processingtemperature range. The heating means can be, for example, a simple hotplate, iron or roller; or hot-air convection heating means; ordielectric heating means.

The described acrylamide polymers can be useful in various layers ofphotothermographic materials known in the art, such as materialsdescribed in U.S. Pat. No. 3,589,903 of Birkeland issued June 29, l97l,U.S. Pat. No. Reissue 26,719 of Sorenson et al. issued Nov. 18, 1969,U.S. Pat. No. 3,429.706 of Shepard et al. issued Feb. 25, I969, U.S.Pat. No. 3,645,739 of Ohkubo et al. issued Feb. 29, 1972, U.S. Pat. No.3,515,559 of Druker et al. issued June 2, l970, and U.S. Pat. No.3,672,904 of DeMauriac issued June 27, l972. The described acrylamidepolymers can be useful in thermographic materials also. Thermographicmaterials in which the acrylamide polymers can be useful are described,for example, in U.S. Pat. Nos. 2,910,377 and 3,094,417 ofworkman issuedJune l8, 1963. Selection of an optimum acrylamide polymer as describedfor use in one or more layers of the photothermographic materials orthermographic materials described in the above art will depend upon suchfactors as the compatability of the acrylamide polymer with the othercomponents of the photothermographic or thermographic material, thedesired image, processing temperature and the like. Preferably, theacrylamide polymers described are used with hydrophilic materials suchas photothermographic materials coated from aqueous formulations.

The following examples are included for a further understanding of theinvention.

EXAMPLE l poly(vinyl alcohol) as a binder 2.15 gJm. hydroquinone l.

The resulting layer was designated layer (1).

The following components were mixed and then coated on layer (1):

poly(vinyl alcohol) as a binder surfactant (Surfactant 106 which is anonylphenoxypolyglycidol sold by Rohm and Haas Co., USA.

total silver as silver iodide and silver complex of3-carboxymethyl-t-methyl-4-thia -Continued ZoIine-Z'thione i) l0% byweight of silver as silver iodide in a gelatino emulsion ii) by weightof silver as a l.6:l (molar ratio) of silver to3-carboxymethyl-4-methyl-4-thiazoline-Z-thione The resulting layer wasdesignated as layer (2).

Poly(acrylamide-co-2-acetoacetoxyethyl methacrylate) (weight ratio 98:2)(inherent viscosity l.40) measured in L0 N NaCl solution at aconcentration of 0.25 g./dl. at a temperature of 25 C. was then coatedon layer (2) at the concentration of 7.26 g. of the acrylamide copolymerper m? of support. The acrylamide copolymer layer was designated layer(3).

The resulting photothermographic element was packaged in a so-calleddouble paper envelope, that is, a black paper envelope inside a yellowpaper envelope. The double paper envelope was stored in a controlledtemperature-relative humidity chamber at 38 C. and 50% relativehumidity. After 7 days and after 14 days, samples of thephotothermographic element were removed from the double paper envelopeand imagewiseexposed for 10 sec. to xenon light in a sensitometer. Thesamples of exposed photothermographic element were uniformly heatedafter exposure by passing them over a metal roller for 4 sec. at C. Adeveloped image was observed in each of the samples. The Dmax, Dmin andphotographic speed for each of the images in the samples were the sameas those of an image developed in an equivalent photothermographicelement with the exception that the imagewise exposure and imagedevelopment were carried out without storing the photothermographicelement for a period of time.

EXAMPLE 2 This is a comparative example.

A control photothermographic element (B) was prepared as in Example 1without the described acrylamide copolymer overcoat (layer 3), and thenimagewiseexposed and developed in the same manner as in Example 1. Thiscontrol photothermographic element provided no acceptable developedimage after storage for 14 days under the described incubationconditions in a double paper envelope.

EXAMPLE 3 EXAMPLES 4-10 A photothermographic element was prepared asfollows.

The following components were mixed and then coated on a resin-coatedpaper support:

pol (vinyl alcohol) as a binder g./m. su actant (Surfactant IOG) Theresulting photothermmographic layer was overcoated with the polymersdesignated in the polymer column of following Table l.

The resulting layer was designated as layer (2).

The resulting photothermographic element was imagewise-exposed totungsten light as described in Examples 4-10 and then overall heated bycontacting the element with a metal block at 160 CV for 4 see. A developed image is observed. The image had the sensitometric propertiesgiven in following Table 2.

EXAMPLE 12 The procedure in Example 1 l was repeated with the exceptionthat an overcoat containing 1.08 g./m. ofpoly(acrylamide-co-2-acetoacetoxyethyl methacrylate) (weight ratio 98:2)was applied to layer (2) prior to imagewise exposure. The resultingsensitometric properties are reported in following Table 2.

The overcoated photothermographic element was T bl 2 imagewise-exposedto tungsten light in a sensitometer l b L E R l for sec. The exposedphotothermographic element gg was overall heated by contacting it with ametal block Example Relative Density Above at 160 C. for 4 sec. Similarsamples of the photothermographic element were incubated in a doublepaper H f h 1'3 rcfuence poi, envelope at 38 C and 50% relative humidityfor up to i )2 3 WEE S 15 days. The resulting samples were then lmagewlse- 12 fresh 08 reference pom exposed and overall heated as above.The Dmin. Dmax 25 1 week 108 .06 and relative photographic speed valuesfor each photo- 2 098 3weeks 1.16 .14 thermographic element are given infollowing Table 1.

Table 1 Example Log E Relative Speed at No. incubation Dmin Dmax 0.6Density Above Dmin Polymer Overcoat 4 fresh 0.08 1.36 reference point noovercoat 7 days 0.07 1.20 0.90 14 days 0.06 1.02 l.07 5 f h L08 L08 MWMnr-rulnmMv-ro-2-mvwnrwt- 7 days 0.05 1.1 1 -0.30 oxyethylmethylacrylate)(weight ratio 98:2) 15 days 0.07 1.28 0.31 (polymer V 6fresh 0.06 1.13 +0.04 2.15 g./rn." of equal parts by weight of poly- 7days 0.10 1.12 +0.04 mer VII and poly(vinyl alcohol) 15 days 0.07 1128+0.04 7 fresh 0.06 1.32 -0.08 double overcoat; first overcoat consistingof 7 days 0.08 1.24 0.04 1.08 g./m. of polymer VII and a top overcoat 14days 0.08 1.30 -0. 12 consisting of 1.08 glm. of poly(vinyl alcohol) 8*fresh 0.09 0.94 same as ref 0.54 g./m. of sulfoethylcellulose 4 days0.13 1.21 +006 13 days 0.05 1118 -0.48 9* fresh 0.16 0.96 +0.02 1.08g./m.'- of sodium cellulose sulfate 4 days 0.08 1.17 +0.11 13 days 0.141.28 -0.04 10 fresh 0.09 1.16 +0.06 2.15 g./m. of equal parts by weightof poly- 4 days 0.09 1.16 +0.06 (acrylamide-co-acrylic acid) andpoly(vinyl 13 days 0.09 1.29 0.18 alcohol) 'CDlTlpill'iillVC exampleEXAMPLE 1 1 EXAMPLE 13 This is a comparative example.

A resin-coated paper support was coated with 1.08 g./m. of polymer Vllcontaining 1% by weight of formaldehyde based on the weight of thepolymer. This was designated as layer (1 On layer 1) was coated aphotothermographic composition containing:

3-mercapto-l .2.4 triazole 0.022 g./rn. isopropylhydroquinone 1.62 g./m.Surfactant 100 0.008 g./m. total silver as silver iodide and 0.81 g./m.

silver complex of 3-carhoxymethyl-4-methyl-4-thiazoline-2- thione i) 10%by weight of silver as silver iodide in a gelatino emulsion ii) 90% byweight of silver as a 1.611 (molar ratio) of silver to 3-carboxymethyl-4-methyl-4thia2oline'2- thione A photothermographic element was preparedas follows:

A resin-coated paper support was coated with a layer comprising amixture of the following:

po1y(acrylamide-co-2acetoacetoxy- 2.42 g./m. ethyl)methacrylate (weightratio tertiary-butylhydroquinone 1 .61 gJm. Surfactant 106 0.008 g./m.

agewise-exposed and then overall heated to develop an image as describedin Example 1.

The sensitometric results of the photothermographic element are given infollowing Table 3, including the results from incubation.

Table 3 lncubation Log E Relative (38 C. at 50% Speed at 0.6 RelativeHumidity Dmax Density Above Dmin fresh 1.2 reference point l week 1.2+06 2 weeks 1.15 .2 3 weeks 1 1 .2

EXAMPLES l425 In each of the following examples, a photothermographicelement was prepared by coating the following composition on aresin-coated paper support at the designated concentrations:

silver iodide gelatino emulsion 0.075 g. Ag/m. silver complex of3-carboxy 0.68 g. Ag/m? methy1-4-methyl-4-thiazo1ine Z-thionetertiary-hutylhydroquinorie 1.88 g./m. 3-mcrcapto1.2,4triazo1e 0.0094gjm. 2.4-dimercuptopyrimidine 0.0024 g./m. Surfactant 106 0.008 g./m.

lmagewise exposure was to tungsten light for 4 seconds and processing ofthe photothermographic elem ment was carried out by overall heating theexposed element for 4 seconds at 155C. The sensitometric results,polymer location and particular polymers employed for Examples 14-25 aredescribed in following Table 4.

15 EXAMPLES 26-37 Table 4 Fresh l Wk./37.8 C./50% RH Polymer Location'Relative Relative Example Uct T-ct Polymer Speed* Contrast Dmin DmaxSpeed* Contrast Dmin Dmax 14 X V 100 1.41 0.03 1.05 25 1.04 0.03 1.03 15X X V 82 1.21 0.05 0.97 16.5 0.77 0.05 0.80 16 X V1 62 1.07 0.02 1.169.1 0.42 0.02 0.45 17 X X V1 31 0.67 0.05 0.90 3.2 0.34 0.05 0.40 18 XV11 89 1.26 0.02 1.06 18.5 0.48 0.02 0.78 19 X X V11 50 1.00 0.05 0.9511.8 0.62 0.07 0.74 20 X V111 94 1.29 0.03 1.12 28 0.65 0.03 0.93 21 X XV111 73 1.09 0.05 1.00 15.5 0.50 0.05 0.66 22 X 1X 69 1.09 0.02 1 160.59 0.02 0.90 23 X X 1X 19.5 0.60 0.05 0.82 6.5 0.42 0.05 0.54 24 X 183 1.08 0.02 1.18 32.5 0.65 0.02 0.93 25 X X 1 63 0.97 0.03 1.06 22.50.57 0.03 0.64

relative speed measured at 0.30 above Dmin. 4 sec. imagcwise exposure totungsten light; processed 4 sec./155C. all polymers coated at 1.08 gjm."U-ct means a coating directly on the support and under thephotothermographic layer; Tct means a coating directly on thephotothcrmugraphic layer Table 5 Fresh 1 Wk./37.8 (750% RH PolymerLocation Relative Relative Example U-ct T-ct Speed Contrast Dmin DmaxSpeed Contrast Dmin Dmax Polymer 26 X 100 1.25 0.01 1.10 28 0.65 0.010.94 V 27 X X 107 1.26 0.07 0.78 57 1.14 0.08 1.02 V 28 X 53 0.91 0.011.06 25 0.70 0.01 0.82 V1 29 X X 29 0.85 0.06 0.93 8.5 0.51 0.07 0.48 V130 X 1.22 0.01 1,05 43 0.86 0.01 1.05 V11 31 X X 89 1.07 0.08 0.76 571.21 0.10 0.90 V11 32 X 95 1.25 0.01 1.06 41 0.75 0.01 1.06 V111 33 X X107 1.15 0.06 0.72 105 1.28 0.09 0.86 V111 34 X 69 1.06 0.01 1.19 270.63 0.01 0.90 1X 35 X X 32.5 1.12 0.06 1.03 20 0.97 0.08 1.08 1X 36 X1.40 0.01 l. 10 54 1.06 0.01 1.20 1 37 X X 107 1.22 0.01 0.84 87 1.400.03 1.10 1

all polymers at 1.011 gjm. "relative speed measured at 0.30 above Dmin 17 EXAMPLE 38 Results similar to Examples 14-25 were observed when 0.43g./m. of polymer Vll was employed as a binder in the photothermographiclayer and the described overcoat layer comprised polymer 1 having theweight ratios of 60:40, 80:20. 90: or 95:5. The sensitometric resultsprovided with these materials are given in following Table 6.

orthosilicate was employed in both the undercoat layer and the overcoatlayer for the photothermographic element according to Example 43. Eachof the photothermographic elements contained 0.43 g. polymer Vll per m?as a binder in the photothermographic layer. Imagewise exposure andprocessing were carried out as described in Example 1 l. Thesensitometric results are given in following Table 8.

Table 8 Fresh l Wk./37.8 C./50% RH Relative Relative Example SpeedContrast Dmin Dmax Speed* Contrast Dmin Dmax Polymar relative speedmeasured at 0.30 above Dmin "poly( 1-vinyl-2-pyrrolidonel (comparativeexample) Table 6 Fresh l Wk./37.8 C./50% RH Relative Relative WeightRatio of Monomers Example Speed" Contrast Dmin Dmax Speed Contrast DminDmax Used to Prepare Polymer 38C 132 1.16 0.0! 1.22 59 1.00 0.01 l.l l(901W) (polymer I) 38D 87 l.ll 0.01 1.12 45 1.22 0.01 0.92 (:5)

all polymers at 1.011 gjm. in overcoat layer; polymer is poly(acrylamideco-l-vinylimidazoie) "relative speed measured at 0.30 above DminEXAMPLES 39-42 The photothermographic element as described in Example 38was used, with the exception that an overcoat layer was used comprising1.08 g./m. of polymer l and a layer was used between the resin-coatedpaper support and the photothermographic layer also comprising 40polymer l. The resulting photothermographic element wasimagewise-exposed and processed as described in Example 14. Thedeveloped image resulting and the sensitometric properties of thedeveloped image are 35 The described polymer (A) useful inphotothermographic materials are most conveniently prepared as solutionpolymers and then used as such without isolation. Some of the resultingpolymer solutions have rather high pH and should be adjusted to thedesired coating pH, usually about pH 4, prior to coating.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifica tions can be effected within the spirit and scope of thegiven in following Table 7. 5 invention.

Table 7 Fresh 1 Wk./37.8 C./50% RH Relative Relative Weight Ratio ofMonomers Example Speed" Contrast Dmin Dmax Speed** Contrast Dmin DmaxUsed to Prepare Polymer 4] 339 1.07 0.02 1.]0 289 L48 0.02 1.26 (901K))(polymer I) 42 316 0.95 0.02 1.00 276 1.60 0.02 1.20 (95.5)

all polymers at 1.08 g./m.'-' in undercoat layer and overcoat layer;formaldehyde is present only in the undercoat layer; the polymer is usedin each of the overcoat and undercoat layers; the polymer ispolytacrylamide-co-l-vinylimidazole] "rclatiu: speed measured at 0.30above Dmin EXAMPLES 43-47 What is claimed is: l. [n a photothermographicelement comprising a support having thereon (l):

a. photographic silver halide in association with b. a silver salt of aheterocyclic thione, said heterocyclic thione being represented by theformula:

wherein R represents atoms completing a 5 member heterocyclic nucleusand Z is alkylene containing 1 to 30 carbon atoms,

c. an organic reducing agent for said silver salt of a heterocyclicthione, and d. a polymeric, synthetic binder and contiguous to (l), atleast one polymer layer (H), the improvement wherein said polymer layer(II) comprises at least 50% by weight of a polymer (A) compris ing therepeating units represented by the formulas:

RI -(-CH2CH+ and +CH C+ l C=O R wherein:

R is hydrogen or alkyl containing 1 to 4 carbon atoms,

R is an imidazolyl, N-substituted carbamoyl, 2-

pyrrolidonyl, acetoacetoxyethoxycarbonyl, acetoacetonylmethylphenyl,ethoxycarbonylaceto, pyridyl, hydroxy, hydroxyalkyl containing 1 to 4carbon atoms, carboxy, carboxyethoxycarbonyl, a heterocyclic ammoniumsalt group having a S- or 6-membered azonia nitrogen-containing ring, ora pyridinium salt group, and

the weight ratio of starting monomers for said units B and C is,respectively. about 60:40 to lz0.

2. A photothermographic element as in claim 1 also comprising in saidlayer (II) at least one polymer other than said copolymer (A),

3. A photothermographic element as in claim 1 wherein said copolymer (A)comprises at least 50% by weight of poly-( acrylamide-co-3-methyllvinylimidazolium methosulfate).

4. A photothermographic element as in claim I wherein said copolymer (A)comprises at least 50% by weight of poly-(acrylamide).

5. A photothermographic element as in claim 1 wherein said copolymer (A)comprises at least 50% by weight of poly-(acrylamide-col-vinylimidazole).

6. A photothermographic element as in claim 1 wherein said copolymer (A)comprises at least 50% by weight of poly-( acrylamide-co-Z-methyllvinylimidazole).

7. A photothermographic element as in claim 1 wherein said copolymer (A)comprises at least 50% by weight ofpoly-(acrylarnide-co-lvinyl-2-pyrrolidone).

8. A photothermographic element as in claim 1 comprising a supporthaving thereon, in sequence:

l. a layer of poly(acrylamide-co-l-vinylimidazole), wherein the weightratio of starting monomers for said copolymer is, respectively, about60:40 to about 98:2.

II. a photothermographic layer comprising:

a. photographic silver iodide in association with b. a silver salt of aheterocyclic thione selected from the group consisting of silver saltsof 3 carboxymethyl-4-methyl-4-thiazoline-2-thione, 3-( Z-carboxyethyl)-4-hydroxymethyl-4- thiazoline-2-thione,3-(2-carboxyethyl)benzothiazoline-Z-thione, 3 2-carboxyethyl )-4-methyl-4-thiazoline-2-thione, and combinations thereof,

c. t-butylhydroquinone, and

d. a poly(acrylamide-co-Z-acetoacetoxyethyl methacrylate orpoly(acrylamide-col vinylimidazole) binder, and

Ill. an overcoat layer of poly(acrylamide-co-lvinylimidazole) whereinthe weight ratio of starting monomers for said copolymer is,respectively, about 60:40 to about 98:2.

9. A photothermographic element as in claim 1 wherein said layer ([1)also comprises a silver halide developing agent.

10. A photothermographic element as in claim I wherein said layer (ll)comprises about 0.40 to about 2.15 g. of said copolymer (A) per in. ofsaid support.

11. A photothermographic element as in claim 1 wherein said polymericsynthetic binder comprises a polymer selected from the group consistingof poly(acrylamide), poly(acrylamide-co-Z-acetoacetoxyethylmethacrylate), poly(acrylamide-co-a-chloroacrylic acid),poly(acrylamide-co-l-vinylimidazole), poly- (vinyl alcohol), andcombinations thereof.

12. A photothermographic element as in claim 1 also comprising ahardener in said layer ([1).

13. A photothermographic element as in claim 1 also comprising aaldehyde hardener in said layer (ll).

14. A photothermographic element as in claim 1 also comprising a3-mercapto-l,2,4-triazole toner.

15. A photothermographic element as in claim 1 also comprising a2,4-dimercaptopyrimidine toner.

16. A process of developing a latent image in a photothermographicelement as defined in claim 1 comprising heating said element to atemperature of about to about 200 C.

17. A process as in claim 16 wherein said photothermographic element isheated for about 0.5 to about 60 sec.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT NO. 3,893,860 Page 1 of 5 DATED 1 July 8, 1975 Richard C. Suttonand Heinz E. Stapelfeldt it is certified that error appears in theab0ve-identified patent and that said Letters Patent are herebycorrected as shown below:

INVENTOR(S) 2 Column 3, line 20, "80" should read ---80C.--.

Column 5, line 25, the formula should read as follows:

Z-COOH Column 6, line 60, "especially" should read s (XXI) T I UNITEDSTATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT N0.3,893, p 2 f 5 a e O DATED July 8, 1975 g lNvENTOkt 1 Richard C. Suttonand Heinz E. Stapelf It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 7, line 15, the formula should read as follows:

6 1 /R -N r 7 =s (XXII) 1 z coorr Column 7, line 35, the formula shouldread as follows:

3 ,R O\ r u t /-=s (XXIII) Y Z COOH Column 8, line 30, "38" should read-38C-- Column 10, line 30, mmethacrylate" should read -methacrylate--.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT N0. 3,893,860 DATED 1 July 8 Page 5 Of 5 INVENTORW 1 Richard c.Sutton and Heinz E. Stapelfeldt It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown betow:

Column ll, line 8, "100" should read ---lOOC.--- Line 9, "1 ro" shouldread ---1 roc.---. Line 12, "150" should read ---l50C.--.

Column l3, line 13, "photothermmographic" should read-photothermographic--.

Column 16, lines 22-23, "particular polymer location" should read---particular polymer and polymer location---.

Column 20, line #0, "a aldehyde" should read --an aldehyde---.

Signed and Sealed this A ties r:

RUTH C. MASON C. MARSHALL DANN Arresting Ofi'r'cer Commissioner ofParents and Trademarks

1. IN A PHOTOTHERMOGRAPHIC ELEMENT COMPRISING A SUPPORT HAVING THEREON(1): A. PHOTOGRAPHIC SILVER HALIDE IN ASSOCIATION WITH B. A SILVER SALTOF A HETEROCYCLIC THIONE, SAID HETEROCYCLIC THIONE BEING REPRESENTED BYTHE FORMULA:
 2. A photothermographic element as in claim 1 alsocomprising in said layer (II) at least one polymer other than saidcopolymer (A).
 3. A photothermographic element as in claim 1 whereinsaid copolymer (A) comprises at least 50% by weight ofpoly-(acrylamide-co-3-methyl-1-vinylimidazolium methosulfate).
 4. Aphotothermographic element as in claim 1 wherein said copolymer (A)comprises at least 50% by weight of poly-(acrylamide).
 5. Aphotothermographic element as in claim 1 wherein said copolymer (A)comprises at least 50% by weight ofpoly-(acrylamide-co-1-vinylimidazole).
 6. A photothermographic elementas in claim 1 wherein said copolymer (A) comprises at least 50% byweight of poly-(acrylamide-co-2-methyl-1-vinylimidazole).
 7. AphotothermoGraphic element as in claim 1 wherein said copolymer (A)comprises at least 50% by weight ofpoly-(acrylamide-co-1-vinyl-2-pyrrolidone).
 8. A photothermographicelement as in claim 1 comprising a support having thereon, in sequence:I. a layer of poly(acrylamide-co-1-vinylimidazole), wherein the weightratio of starting monomers for said copolymer is, respectively, about60:40 to about 98:2. II. a photothermographic layer comprising: a.photographic silver iodide in association with b. a silver salt of aheterocyclic thione selected from the group consisting of silver saltsof 3-carboxymethyl-4-methyl-4-thiazoline-2-thione,3-(2-carboxyethyl)-4-hydroxymethyl-4-thiazoline-2-thione,3-(2-carboxyethyl)benzothiazoline-2-thione,3-(2-carboxyethyl)-4-methyl-4-thiazoline-2-thione, and combinationsthereof, c. t-butylhydroquinone, and d. apoly(acrylamide-co-2-acetoacetoxyethyl methacrylate orpoly(acrylamide-co-1-vinylimidazole) binder, and III. an overcoat layerof poly(acrylamide-co-1-vinylimidazole) wherein the weight ratio ofstarting monomers for said copolymer is, respectively, about 60:40 toabout 98:2.
 9. A photothermographic element as in claim 1 wherein saidlayer (II) also comprises a silver halide developing agent.
 10. Aphotothermographic element as in claim 1 wherein said layer (II)comprises about 0.40 to about 2.15 g. of said copolymer (A) per m.2 ofsaid support.
 11. A photothermographic element as in claim 1 whereinsaid polymeric synthetic binder comprises a polymer selected from thegroup consisting of poly(acrylamide),poly(acrylamide-co-2-acetoacetoxyethyl methacrylate),poly(acrylamide-co- Alpha -chloroacrylic acid),poly(acrylamide-co-1-vinylimidazole), poly-(vinyl alcohol), andcombinations thereof.
 12. A photothermographic element as in claim 1also comprising a hardener in said layer (II).
 13. A photothermographicelement as in claim 1 also comprising a aldehyde hardener in said layer(II).
 14. A photothermographic element as in claim 1 also comprising a3-mercapto-1,2,4-triazole toner.
 15. A photothermographic element as inclaim 1 also comprising a 2,4-dimercaptopyrimidine toner.
 16. A processof developing a latent image in a photothermographic element as definedin claim 1 comprising heating said element to a temperature of about 80*to about 200* C.
 17. A process as in claim 16 wherein saidphotothermographic element is heated for about 0.5 to about 60 sec.